1. Field of the Invention
This invention relates to non-stick fluoropolymer finishes exhibiting improved abrasion resistance.
2. Description of Related Art
The technology of forming a non-stick fluoropolymer finish on a substrate, such as aluminum, to form a release surface for such applications as cookware has typically involved the use of at least two coatings on the substrate, an undercoat typically called a primer to adhere to the substrate, and an overcoat to provide the non-stick coating (finish). Because of the non-stick nature of fluoropolymer, the primer is formulated to contain one or more adhesion promoters. Colloidal silica and thermally stable polymer binder (generally fluorine-free) are examples of adhesion promoters, as disclosed in U.S. Pat. No. 5,562,991 to Tannenbaum. The adhesion promoter constitutes a substantial portion of the primer, typically at least 40 wt %, based on the combined weight of fluoropolymer plus adhesion promoter. The surface of the overcoat is rich in, if not entirely, fluoropolymer. The presence of fluoropolymer both in the primer and the overcoat promotes adhesion between these coatings (intercoat adhesion).
It has become known to incorporate relatively large particles of inorganic, non-metallic particles into the primer layer to increase abrasion resistance of the non-stick finish. EP 0 724 915 to Bignami discloses the use of cristobilite, which is a SiO2 mineral having a Moh's hardness of 6.5 (corresponds to a Knoop hardness of 820), in the primer layer. U.S. Pat. Nos. 6,291,054 to Thomas et al. and 6,761,964 to Tannenbaum disclose the use of harder large ceramic particles, preferably having a Knoop hardness of at least 1200, in the primer layer. In Example 1 of '054, a single coat system is disclosed in which the single coat is quite thick (19.5-59.7 micrometers) and the amount of polymer binder exceeds the amount of fluoropolymer. In U.S. Pat. No. 6,863,974 to Shah et al., no fluoropolymer is present in the primer layer, just heat resistant adhesion promoter (polymer binder) along with inert inorganic particles (ceramic particles) having an average particle size of at least 10 micrometers and Moh's hardness of at least 5. Intercoat adhesion to the topcoat is obtained by including adhesion promoter in the fluoropolymer-containing topcoat composition.
While at least the '054 and '942 patents have provided improvement in abrasion resistance on a commercial basis, further improvement in abrasion resistance is desired.